Method for selective decarburization of alloy steels

ABSTRACT

A method for selective decarburization of crude, molten alloy steel by means of injecting into the bath, peripherally of a submerged tuyere, fuel oil which is maintained in the liquid state at least to the tuyere-bath interface and simultaneously injecting a mixture of oxygen and vaporized fuel oil axially through said tuyere into the bath until the carbon-alloy equilibrium is reached, and thereafter adjusting the mixture so as to be rich in vaporized fuel oil and continuing said fuel oil enriched injection until the desired carbon level is reached and thereupon discontinuing fuel oil and gaseous mixture injections and purging undesired hydrogen from said bath by injecting both axially and peripherally of said tuyere an inert gas.

United States Patent 1191 Cox et al.

[ June 24, 1975 METHOD FOR SELECTIVE DECARBURIZATION OF ALLOY STEELS[73] Assignee: Koppers Company, Inc., Pittsburgh,

[22] Filed: June 7, 1973 [2|] Appl. No.: 367,839

3,330,645 7/1967 DcMousticr 3.706.549 12/1972 Knuppel 75/60 3,725,0414/[973 Ramachandran....................... 75/60 8/l973 l2/l974 Knuppcl75/60 Leroy 75/60 Primary Examiner-P. D. Rosenberg Attorney, Agent, orFirm-Robert D. Yeager I 5 7 ABSTRACT A method for selectivede'carburization of crude, molten alloy steel by means of injecting intothe bath, peripherally of a submerged tuyere, fuel oil which ismaintained in the liquid state at least to the tuyerebath interface andsimultaneously injecting a mixture of oxygen and vaporized fuel oilaxially through said tuyere into the bath until the carbon-alloyequilibrium is reached, and thereafter adjusting the mixture so as to berich in vaporized fuel oil and continuing said fuel oil enrichedinjection until the desired carbon level is reached and thereupondiscontinuing fuel oil and gaseous mixture injections and purgingundesired hydrogen from said bath by injecting both axially andperipherally of said tuyere an inert gas.

4 Claims, 2 Drawing Figures 0X YGEN METHOD FOR SELECTIVE DECARBURIZATIONOF ALLOY STEELS FIELD OF THE INVENTION The present invention relates toa method for the production of ferro-alloy steels and, in particular, toa method for the selective decarhurization of high chromium steels andthe like.

BACKGROUND OF THE INVENTION In the refining of iron into steel, thedesirability of utilizing oxygen to oxidize the impurities in the molteniron, particularly carbon, is well known. It has also become generallywell known that substantially pure oxygen is useful in thedeearburization of chromium steels. However, the amount of carbon thatcan be removed from a molten bath of chromium steel is limited by theequilibrium relationship between carbon, chromium and bath temperature.Attempts to remove carbon beyond the levels established by theequilibrium relationship result in the oxidation of chromium and othermetal alloys and, thus, their consequential removal from the bath intothe slag. Accordingly, a number of methods have been devised to obtaincarbon levels below those established by the equilibrium relationship.

One of the most common methods is the utilization of ferro-alloyadditives containing silicon. The addition of the additives to the meltafter decarburization reduces the chromium and other metal oxidesexisting in the slag and returns them to the melt. While this method isvery effective, it is also extremely expensive. Alternatively,decarburization has been carried out at higher temperatures toeffectuate a shift in the equilibrium point resulting in a lower carbonlevel but these higher temperatures have also caused a substantialincrease in refractory wear. Another method typically used todecarburize the melt is to reduce the partial pressure of the meltsystem by means of vacuum degassing.

The most significant improvement has been methods for preferentiallyoxidizing carbon during decarburization by utilization of anoxygen/argon mixture. Illustrative of these processes are US. Pat. Nos.3,046,l07 and 3,252,790. These processes generally utilize an electricarc furnace as a scrap and alloy melter to prepare a crude high-carbon,high-chromium hot metal which is thereafter converted in a refiningvessel to the desired stainless steel. The decarburization of the crudemelt follows the well known equilibrium relationship that exists betweencarbon, chromium and temperature. These processes are able to shift theequilibrium relationship by adding an inert gas to the reactant oxygenwhereby the carbon monoxide formed by oxidation of carbon has a lowerpartial pressure than it has when oxygen alone is used. Thus, byreducing the partial pressure of the formed carbon monoxide, a lowercarbon content can be obtained for a given chromium content andtemperature level. The equilibrium condition of the melt: C MO 7- M +CO,where M is the metal in the melt, O is the oxygen in the melt and C isthe carbon, it is shifted to the right.

This operation has gained general acceptance because of the highhomogeneity of the steel produced as well as its quality.Notwithstanding the advantages of this method, certain disadvantagesexist. The utilization of argon as an oxygen diluent is expensive evenwhere nitrogen is used during a portion of the blow. Moreover, extremelyrapid wear is experienced by the tuyeres and the refractory bottomsadjacent the tuyeres caused by the high heat and/or ferric oxidegenerated by blowing pure oxygen. The tuyere and refractory wear issimilar to the wear experienced by Thomas and Bessemer convertersblowing substantially pure oxygen.

Early efforts with Thomas and Bessemer producers to cure the problem ofrapid deterioration of the bottorn by cooling the tuyeres during theoxygen blow produced no commercially practical results. Examples ofthese efforts include Lellep US. Pat. No. 2,333,654 wherein the tuyereswere cooled by circulating water in an annular duct around the oxygensupply pipe. Copper tubes were used to form tuyeres in converter bottomsrather than employing conventional tuyeres formed directly in theconverter bottom, see Kosmider et al. U.S. Pat. No. 2,829,879. in thisarrangement, the high heat conductivity of the copper was used to coolthe tuyeres. Variations of the Kosmider et al. concept were attempted bySavard et al. US. Pat. No. 2,855,293 and Compagnie des Ateliers etForges de la Loire French Pat. No. 1,503,756. Recent attempts toovercome erosion of the tuyere zones in a converter bottom blown withoxygen involve the use of a double tuyere, generally comprisingconcentric pipes located in the converter bottom. Gases such as steam orcarbon dioxide were blown through the outer annulus to shield" theoxygen jet blown through the axial pipe, see Westfalenhutte French Pat.No. 1,058,] 8]; and Luxemburg Pat. No. 3,397,878. Hydrocarbon gases havealso been used to shield the oxygen jet, French Pat. No. 1,450,718. Adifferent approach has been taken effectively to increase tuyere and/orrefractory life, Offenlegunsschift No. 2,033,975, but it has not beendemonstrated to be applicable to the production of high chromium alloysteels. Other methods which sought to protect the refractory wear aswell as the tuyeres such as in US. Pat. Nos. 3,330,645 and 3,490,755, inwhich porous refractory sheaths were utilized to percolate gas at theinter face of the molten metal did not result in commercially adoptedprocesses.

Accordingly, it is an object of the present invention to provide amethod in which high chromium alloy steels can be made by utilizingsubmerged oxygen blowing without the deleterious effects commonlyassociated therewith. It is thus an object of the present invention toprovide a method for oxygen deearburization of alloy steels without theuse of expensive argon as a diluent and without high tuyere andrefractory wear generally associated with oxygen decarburization.

SUMMARY OF THE INVENTION The present invention provides a method forcooling a submerged tuyere in a refining vessel and for reducing thepartial pressure of carbon monoxide in the melt to obtain low carbonlevels. The method of the present invention provides a selectivedecarburization of a crude chromium alloy steel melt preferably preparedin an electric arc furnace.

Generally, the method comprises injecting a mixture of oxygen andvaporized fuel oil through the axial portion of a concentric tuyere forthat period which is sufficient to achieve the theoreticalcarbon-chromiumtemperature equilibrium of the melt. Simultaneously withthe axial injection, fuel oil, maintained in the liquid state at leastto the tuyere-bath interface, is injected into the refining vesselperipherally of the axial portion of the tuyere. When the theoreticalcarbon-chromiumtemperature equilibrium is reached, the volumetric ratioof vaporized fuel oil to oxygen in the gaseous mixture is increased toat least between 2:] and 6:] until the melt is decarburized to thedesired carbon level. The peripheral fuel oil injection and gas mixtureinjection are stopped when the desired end point is reached, and aninert gas, such as carbon dioxide, argon and the like, is injected for ashort time into the melt to purge the melt of any residual hydrogen.

More specifically, the present invention provides a process in which ahigh-carbon, high-chromium crude melt is prepared and deslagged in anelectric arc furnace. Decarburization is carried out in a refiningvessel having tuyeres submerged below the bath level and located,preferably, in the bottom thereof. The tuyeres are of a double orconcentric tube configuration in which a liquid hydrocarbon, preferablyfuel oil, is injected into the molten bath through a space between theperipheral and axial tubes, and a mixture of oxygen and vaporized liquidhydrocarbon is injected through the central or axial tube.

The liquid hydrocarbon injected through the peripheral tube ismaintained in the liquid state at least to the tuyere-bath interface. Bymaintaining the hydrocarbon in the liquid state, that is preventingvaporization within the tuyere, the heat of vaporization as well as theendothermic heat of hydrocarbon cracking is available to cool the tuyeretip. The cracking may, under optimum conditions, provide preferentialcarbon donating residuum to the melt adjacent the tuyere. The carbonresiduum is believed to act as a protective film or insulator betweenthe melt and the refractory, but whether or not any carbon residuum isdeposited the cracking will preferentially donate carbon to and effectreduction of the ferric oxide which is known to have the greatestcorrosive effect upon the lining. Accordingly, by preventingvaporization of the liquid hydrocarbon within the tuyere it is possibleto both protect the tuyere tip and the adjacent refractory material,thus substantially reducing the rapid wear experienced when blowing pureoxygen into the molten bath.

Generally, the decarburization is carried out in three stages. In thefirst stage, preferably oxygen alone, or a mixture of oxygen andvaporized liquid hydrocarbon is injected into the bath through the axialtuyere. Where a gaseous mixture is used, the liquid hydrocarbon isvaporized upon addition to the oxygen to provide an oxygen tohydrocarbon vapor volumetric ratio of at least 3:1 or 4:1. The secondstage injection is commenced when the theoreticalcarbon-chromium-temperature equilibrium is reached as calculated uponthe basis of the amount of oxygen injected in the first stage and ananalysis of the melt. During the second stage the volume of vaporizedhydrocarbon to oxygen is preferably about 3 to 4:1. Other ratios thatcan be employed for second stage blowing, range from 2:1 to about 6:1;but at the lower ratios, a higher percentage of chromium is oxidizedthan is generally preferred, and at the higher ratios the refining timeis increased although very low carbon levels are achieved. The secondstage is continued until the desired carbon content in the melt isreached. At the desired end point, the flow of both the oxygen-hydrocarbon mixture and the peripheral hydrocarbon cooling liquid isdiscontinued. An inert gas,

preferably carbon dioxide or argon, is injected through the axial andperipheral circuits of the tuyere. The third stage injection of inertgas is continued for a period of from about I to 3 minutes to reduce anydissolved hydrogen in the melt as well as to deoxidize the melt.

While the process is well suited to stepwise operation, it is clear thatthe process can be practiced on the basis of a continual ratioadjustment without utilizing separate and discrete steps. In such acase, the desired volumetric ratios are applicable at the analogoustimes during refining.

The liquid hydrocarbon in the oxygen stream acts as a diluent for thecarbon monoxide formed during decarburization of the crude melt. It isbelieved that the molecules of the liquid hydrocarbon, fuel oil, uponvaporization in the oxygen stream within the tuyere, are activated bythe high temperatures prevailing at the interface between the tuyere andmolten bath. The hydrocarbon and oxygen combine to form peroxides whichdecompose to water and aldehyde. This combination and decomposition isin extremely rapid transformation and is believed to occur almostsimultaneously. The aldehyde very rapidly thereafter is converted tocarbon dioxide and water, the latter of which dissociates into hydrogenand oxygen. The carbon di oxide and the hydrogen from the dissociatedwater vapor act as diluents for the carbon monoxide formed in the meltduring decarburization.

In the first stage of the process, the amount of hydrocarbon required issubstantially less than the amount of oxygen required even though thepartial pressure of carbon monoxide does increase with increasing oxygenavailability. The amount of oxygen supplied is substantially the same asthat heretofore required to equilibrate the carbon-chromium-temperatureequilibrium relationship. Since a reduction in the carbon belowequilibrium requires a shift in the equilibrium point by a reduction inthe partial pressure of the carbon monoxide in the bath, the secondstage involves a substantial increase in the volumetric ratios ofvaporized liquid hydrocarbon to oxygen, for example, up to 6:1. It ispreferred, however, that in the second stage the volumetric ratio of thevaporized liquid hydrocarbon to oxygen is 3:1.

Utilizing a liquid hydrocarbon both for oxygen dilution as well as acooling agent for the submerged tuyere, increases the availability ofhydrogen for dissolution into the bath. Generally, a large portion ofthe available hydrogen becomes dissolved within the melt which ifpermitted to remain would render the heat unusable. To remove thedissolved hydrogen to levels of less than 12 PPM for 300 seriesstainless steel, for example, a third stage injection of an inert gas isused to flush the melt. This flushing also has the advantage of loweringthe amount of oxygen dissolved therein.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a cross-sectional plan viewof a tuyere for use with the present invention; and

FIG. 2 is a sectional elevation taken along lines II-ll of FIG. 1 ofsaid tuyere.

PRESENTLY PREFERRED EMBODIMENT The method of selective decarburizationof alloy steel is preferably carried out as a duplexing operation. Theprocess is particularly well adapted for the production of steels havingchromium content in excess of 5% and, preferably, from to 25%, with thecarbon and silicon each present in amounts less than 1%. By practicingthe methods of the present invention, the carbon content can be reducedto around 0.2% without any substantial oxidation and loss of chromium orother alloys.

Preferably, an electric arc furnace is used to melt down scrap and alloyunder dead melt conditions and to bring the melt temperature up tobetween l600 and l700C. The molten metal is then tapped into a ladle,sampled and deslagged. The deslagged crude hot metal is then transferredinto a reaction or refining vessel. The refining vessel can be aconventionally shaped Bessemer type converter adapted for rotation tofacilitate charging, sampling and tapping. The converter is rotatableabout a pair of trunnions which are designed to permit the passage ofoxygen and fuel oil conduits therethrough. The bottom of the converteris fitted with a number of concentric tuyeres, for example, a [00 tonsteel alloy converter would be provided with about five tuyeres.

Referring to FIGS. 1 and 2, tuyere 10 is representative of the type oftuyere preferred for use in the pres ent invention and includes an axialtube 11 and a concentrically aligned outer tube 12. Tuyere 10 is adaptedto be mounted through refractory bottom 14 of the converter by mountingmeans 16. Axial tube 11 has a plurality of fluted passages 17 machinedalong the length of its outer surface. Passages 17 are designed toregister with the inner surface of outer tube 12 to form a plurality ofperipheral passages about the periphery of the axial tube for theperipheral injection of liquid hydrocarbon.

Outer tube 12 is provided with a number of grooves 18 machined along aportion of the inner surface and designed to register with the outersurface of inner tube 11, but not with the fluted passages 17. Grooves18 extend from the base of tube 12 to a position near the tip of tuyere10, for example, within about one inch of the tuyere-bath interface. Thetip ends of grooves 18 are positioned to register with openings 19 whichare angularly formed through axial tube 11 and provide for introductionof fuel oil from the grooves 18 into the oxygen stream in axial tube 11.

The inner diameter of axial tube 11 is preferably between 0.25 to 0.30inches and is made ofa high thermally conductive material such as copperor copper alloys. Outer concentric tube 12, on the other hand, can befabricated from any suitable material such as stainless or plain carbonsteel. The inner diameter of outer tube 12 and the outer diameter ofinner tube 11 are essentially the same so that a tight nestingrelationship therebetween is established to provide the appropriatesealing between passages 17 and grooves 18. Passages 17 are connected toa source of fuel oil by means of line 21. Elongated grooves 18 areconnected at the base of tuyere 10 to line 22 that is connected to anindependently regulatable source of fuel oil. Axial tube 11 is connectedto line 23 that is connected to a source of high pressure oxygen.

During the operation of the present invention, fuel oil is injectedthrough line 21 and passages 17 peripherally of the axial oxygen tuyere.Pressure and/or flow rate in lines 21 and passages 17 is maintained toprevent the fuel oil from vaporizing before it reaches the tuyerebathinterface. Premature vaporization can be detected by fluctuations in theflow meters on lines 21. Fuel oil in line 22, on the other hand, ismaintained at a pressure and flow rate which provides both proper fueloiloxygen vapor mixture ratio and preferably for vaporization of thefuel oil at the end of elongated grooves 18.

By maintaining the fuel oil in a liquid state at least to thetuyere-bath interface, the tuyeres as well as the adjacent refractorylining 14 are protected from the rapid erosion of the bath. It isbelieved that both the heat of vaporization as well as the phenomenasimilar to cracking is used to absorb bath heat and to thereby cool thetuyere. Moreover, the cracked hydrocarbon fuel oil provides acarbonaceous layer or film in the area of the tuyere which either actsas a physical shield against the bath or preferably donates carbon tothe ferric oxide. The fuel oil is preferably maintained at a pressure ofbetween and psi.

Oxygen supplied by line 23 and vaporized fuel oil from elongated groove18 in tuyere 10 are injected into the bath through the axial tubes 11 oftuyere 10. In a lOO-ton heat of a 300 series alloy steel for example,the total oxygen flow rate would, preferably, be about 1000 cubic feetof oxygen per hour per ton with 350 to 400 cubic feet of fuel oil perhour per ton of steel in the first stage. Since the first stage blowlasts until the theoretical carbon-chromium equilibrium point isreached, it is possible to blow in the first stage without theutilization of any fuel oil. The second stage preferably utilizesapproximately 1000 to I200 cubic feet of fuel oil per hour per ton ofsteel and about 300 to 400 cubic feet of oxygen. Number 2 fuel oil, forexample, injected into tuyere 10 at 100 psi will provide approximately10 to l 1 cubic feet per gallon. The second stage blow is ended when thedesired carbon level has been reached. The third stage blow of carbondioxide or argon is utilized to stir the bath and remove any undesiredhydrogen. The third stage blow is from about I to 10 minutes andpreferably about 3 minutes in duration, depending upon the amount ofhydrogen dissolved in the bath.

While presently preferred embodiments of the invention have been shownand described in particularity, it may otherwise be embodied within thescope of the appended claims.

What is claimed is:

l. A method of selective decarburization ofa chromium-alloy steel melt,said decarburization being carried out in a refining vessel having atleast one double tuyere submerged below the surface of the melt, saidmethod comprising: injecting into said melt through a central tube ofsaid tuyere a gas selected from the group consisting of oxygen and amixture of oxygen and fuel oil for a period of time sufficient to reachthe carbon-chromium-temperature equilibrium, while injecting, during theentire said period, fuel oil maintained in the liquid state to thetuyere-melt interface into said melt through said tuyere andperipherally of said central tube; injecting through said central tubeinto said melt a mixture of fuel oil and oxygen in a volumetric gasratio offrom about 2:l to 6: l until said melt is decarburized, whilecontinuing said peripheral flow of fuel oil into said melt; andthereafter discontinuing the injection of fuel oil and said mixture offuel oil and oxygen, and injecting into said melt an inert gas to purgethe melt of hydrogen.

2. A method as set forth in claim 1 wherein oxygen is injected into saidmelt through said central tube for a period of time sufficient to reachthe carbon-chromium-temperature equilibrium.

3. A method as set forth in claim 1 wherein said ratio of fuel oil tooxygen after equilibrium is 32L 4. A method as set forth in claim 1wherein said inert gas is carbon dioxide.

=r a: a

1. A METHOD OF SELECTIVE DECARBURIZATION OF A CHROMIUMALLOY STEEL MELT,SAID DECARBURIZATION BEING CARRIED OUT IN A REFINING VESSEL HAVING ATLEAST ONE DOUBLE TUYERE SUBMERGED BELOW THE SURFACE OF THE MELT, SAIDMETHOD COMPRISING INJECYING INTO SAID MELT THROUGH A CENTRAL TUBE OFSAID TUYERE A GAS SELECTED FROM THE GROUP CONSISTING OF OXYGEN AND AMIXTURE OF OXYGEN AND FUEL OIL FOR A PERIOD OF TIME SUFFICIENT TO REACHTHE CARBON-CHROMIUM-TEMEPRATURE EQUILIBRIUM, WHILE INJECTING, DURING THEENTIRE SAID PERIOD FUEL OIL MAINTAINED IN THE LIQUID STATE TO THETUYERE-MELT INTERFACE INTO SAID MELT THROIGH SAID TUYERE ANDPERIPHERALLY OF SAID CENTRAL TUBE; INJECTING THROUGH SAID CENTRAL TUBEINTO SIAD MELT A MIXTURE OF FUEL OIL AND OXYGEN IN A VOLUMETRIC GASRATIO OF FROM ABOUT 2:1 TO 6:1, UNTIL SAID MELT IS DECARBURIZED, WHILECONTINUING SAID PERIPHERAL FLOW OF FUEL OIL INTO SAID MELT; ANDTHEREAFTER DISCONTINUING THE INJECTION OF FUEL OIL AND SAID MIXTURE OFFUEL OIL AND OXYGEN,
 2. A method as set forth in claim 1 wherein oxygenis injected into said melt through said central tube for a period oftime sufficient to reach the carbon-chromium-temperature equilibrium. 3.A method as set forth in claim 1 wherein said ratio of fuel oil tooxygen after equilibrium is 3:1.
 4. A method as set forth in claim 1wherein said inert gas is carbon dioxide.